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Abstract

Debate on the adaptive origins of primates has long focused on the functional ecology of the
primate visual system. For example, it is hypothesized that variable expression of short- (SWS1) and
middle-to-long-wavelength sensitive (M/LWS) opsins, which confer color vision, can be used to infer
ancestral activity patterns and therefore selective ecological pressures. A problem with this approach is
that opsin gene variation is incompletely known in the grandorder Euarchonta, i.e., the orders Scandentia
(treeshrews), Dermoptera (colugos), and Primates. The ancestral state of primate color vision is therefore
uncertain. Here we report on the genes (OPN1SW and OPN1LW) that encode SWS1 and M/LWS opsins
in seven species of treeshrew, including the sole nocturnal scandentian Ptilocercus lowii. In addition, we
examined the opsin genes of the Central American woolly opossum (Caluromys derbianus), an enduring
ecological analogue in the debate on primate origins. Our results indicate: 1) retention of ultraviolet (UV)
sensitivity in C. derbianus and a shift from UV to blue spectral sensitivities at the base of Euarchonta; 2)
ancient pseudogenization of OPN1SW in the ancestors of P. lowii, but a signature of purifying selection in
those of C. derbianus; and, 3) the absence of OPN1LW polymorphism among diurnal treeshrews. These
findings suggest functional variation in color vision of nocturnal mammals and a distinctive visual
ecology of early primates, perhaps one that demanded greater spatial resolution under light levels that
could support cone-mediated color discrimination.